Although the release of intracellular toxins after chlorination has been reported, the relation between cell lysis and the release and degradation of toxins during chlorination has not been well illustrated. This study used Microcystis aeruginosa to investigate the release and degradation behaviors of toxins after its exposure to chlorine at different doses for prolonged contact time. Scanning electron microscopy (SEM) analysis indicates no considerable change in the morphology for most algae cells. However, the release of intracellular K(+) and intracellular organic matter (IOM) did occur, and the significant K(+) release of 63% was observed even at chlorine exposure as low as 0.8mg/Lmin. The damage of cell membrane was faster than the release of intracellular MCLR at all chlorine doses. Extracellular MCLR accumulated as a result of the fact that intracellular MCLR released from damaged cells faster than it was degraded by chlorine, especially at low dose of chlorine (0.8mg/L). A maximum level of extracellular toxin at a contact time of 60, 30 and 1min was observed at chlorine doses of 1.2, 1.6 and 2.0mg/L, respectively. In addition, Cl(2) consumption (%), toxin release and degradation (%), and cells lysis (%) were well related.